1. Field of the Invention
This invention relates generally to the fabrication of pagewidth reading or writing bars, and more particularly to the fabrication process for a pagewidth linear array of reading or writing bars from subunits, so that tolerance stackup is avoided. By example, illustration of the specific details of the invention will be provided for a pagewidth thermal ink jet printhead array fabricated from fully functional subunits.
2. Description of the Prior Art
It is well known in the reading and/or writing bar industry to assemble pagewidth raster input scanning (RIS) and raster output scanning (ROS) bars from relatively short RIS/ROS subunits placed end-to-end. Once assembled, the pagewidth RIS/ROS bars or reading and writing bar arrays have the requisite length and number of image processing elements to scan or to write an entire line of information at once with a high image resolution. The subunits have either image reading arrays which comprise a succession of image sensing elements to convert the image line into electrical signals or pixels, or image writing arrays which comprise a succession of light producing or other elements employed to produce images in response to an image signal or pixel input.
The prior art has failed to provide a means for fabricating a pagewidth scanning or writing bar array from subunits which has adequate precise alignment tolerance in X, Y, and .crclbar. space which is commercially (i.e. economically) feasible. The prior art solutions to overcome this inability to provide cost effective pagewidth reading or writing bar arrays include optical and electrical arrangements for overlapping several short arrays and abutting short arrays together end-to-end. However, none of these attempts have met with any great degree of success. For example, in the case of abutting smaller arrays together, losses and distortions of the pagewidth image often occurs because of the inability to achieve exact alignment of the smaller arrays with respect to each other. Another important problem with simply abutting chips or subunits is that chip or subunit width errors accumulate over the length of the pagewidth array.
In particular, thermal ink jet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The use of an array of printhead subunits is appropriate because pagewidth printheads cannot be practically fabricated on a single wafer. Full width printbars composed of collinear arrays of thermal ink jet printhead subunits have a number of architectural advantages over staggered offset printbar architecture. One convenient method of fabricating a collinear subunit printbar is to simply butt each printhead subunit up against its neighboring printhead subunit. This fabrication method provides very positive positioning of the printhead subunits and minimizes the nozzle gap between adjacent printhead subunits, but does not prevent tolerance stackup as the pagewidth device is fabricated.
U.S. Pat. No. Re.32,572 to Hawkins et al. discloses several methods for fabricating small ink jet printheads, each printhead being composed of two parts aligned and bonded together. One part is a silicon wafer having a substantially flat substrate with a surface containing a linear array of heating elements and addressing electrodes, and the second part is another silicon wafer having a substrate containing at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together and a linear array of parallel grooves which communicate with the recess and are used as ink jet nozzles. After the bonding, the two wafers are diced into many different printheads with nozzles located in the printhead sides. A number of printheads can then be fixedly mounted in a pagewidth configuration which confronts a moving recording medium for pagewidth printing.
U.S. Pat. No. 4,789,425 to Drake et al. discloses a process for fabricating small ink jet printheads with nozzles located in the printhead roofs.
U.S. Pat. No. 4,774,530 to Hawkins discloses a thick insulative layer sandwiched between the two wafers of the printhead with recesses patterned in it to expose the heating elements to the ink and to provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels.
U.S. Pat. No. 4,759,675 to Bond et al. discloses an apparatus for removing selected integrated die from a wafer array which sequentially moves above the wafer and knocks down die from the array of die into a receptacle for further processing.
U.S. Pat. No. 4,829,324 to Drake et al. discloses a large array ink jet printhead fabrication process for precision assembly with subunits. One embodiment involves abutting edges of subunits having surfaces which follow the {111} planes of a silicon wafer from which they are produced. Another embodiment is disclosed in which, before dicing and abutting, an etched silicon channel wafer is aligned and bonded to an etched silicon heater wafer so that the {111} plane surface of the channel wafer is coplanar with the {111} plane surface of the heater wafer groove.
U.S. Pat. No. 4,822,755 to Hawkins et al., U.S. Pat. No. 4,900,283 to Fukae, and U.S. Pat. No. 4,976,802 to LeBlanc disclose processes for bonding subunits into arrays.
U.S. Pat. No. 4,911,598 to Sarvary et al. discloses a robotic assembly apparatus that places component parts on a workpiece.
U.S. Pat. No. 4,999,077 to Drake et al. discloses a method for fabricating a coplanar full width scanning array from a plurality of relatively short scanning subunits for reading and writing images. The subunits are fixedly mounted in an end-to-end relationship on a flat structural member with the subunit surfaces containing the scanning elements all being coplanar even though at least some of the subunits have varying thickness. This is accomplished by forming from a photopatternable thick film layer one or more keys on the subunit surface having the scanning elements and associated circuitry and positioning the keys into keyways produced from a photopatternable thick film layer on a flat surface of an alignment fixture. A conformal adhesive bonds a structural member to the assembled subunits to form the full width scanning array.
U.S. Pat. No. 4,980,971 to Bartschat et al. discloses a method and apparatus for precision semiconductor chip placement on a silicon substrate including a robotic arm. A television camera is carried by the arm and serves to capture the image of a substrate to locate datum positions. A second camera, stationary with respect to the robotic arm, captures the image of a chip by observing its bottom. A machine vision system processes output signals from the cameras, precisely locates the different types of chips, and controls the robotic arm. Each chip is placed in its precise location of the integrated circuit. The Bartschat et al. patent does not involve abutting subunits or arrays.
A significant drawback to butted arrays of collinear printheads is the effect of tolerance stackup on multiple bar systems. For example, if a series of printhead modules to be butted together are all slightly undersized by two microns, then twenty of these butted modules will result in a printbar that is 40 microns undersized. When multiple pagewidth printbars are to be aligned, as is necessary for a four bar color machine, such an undersized amount is unacceptable. If the droplet ejecting nozzles of the bars are not properly aligned, then the second color droplets from the second bar will not line up with the first color droplets from the first bar and the final image will not properly blend. This is a problem always encountered where multiple pagewidth bars or arrays are each assembled from subunits and used in the field of reading and/or writing bars. This is especially a problem for pagewidth multicolor ink jet printheads assembled from subunits, where droplets of one printhead must align within a given tolerance with the droplets from one or more other printheads.